the.com/grandfather

means The father of one's father or mother; a male grandparent.

from A plain old compound, stacked like generations: 'grand' came into English from Old French 'grant' (great, big — from Latin 'grandis'), and was bolted onto 'father' to mean a father at one remove, a 'great' father. The same logic gives us grandmother, grandson, and so on — 'grand' here means not splendid but elevated by a generation. The French influence after the Norman Conquest layered this 'grand' onto native English kin words, which is why we say grandfather rather than something purely Germanic.